Certain monoclonal antibodies (mAb) directed against CD4 can efficiently block HIV-1 replication in vitro. to explore CD4-directed passive immunotherapy for prevention or treatment of AIDS-virus infection, we previously examined the biological activity of a nondepleting CD4-specific murine mAb, mu5A8. This mAb, specific for domain 2 of CD4 blocks HIV-1 replication at a post-gp120/CD4 binding step. When administered to normal rhesus monkeys, all CD4+ target cells were coated with antibody, yet no cell clearance or measurable immunosuppression occurred. However, strong anti-mouse Ig responses rapidly developed in all monkeys. In the present study, we report a successfully humanized form of mu5A8 (hu5A8) that retains binding to both human and monkey CD4 and anti-AIDS virus activity. When administered intravenously to normal rhesus monkeys, hu5A8 bound to all target CD4+ cells without depletion and showed a significantly longer plasma half-life than mu5A8. Nevertheless, an anti-hu5A8 response directed predominantly against V region determinants did eventually appear within two to four weeks in most animals. However, when hu5A8 was administered to rhesus monkeys chronically infected with the simian immunodeficiency virus of macaques, anti-hu5A8 antibodies were not detected. Repeated administration of hu5A8 in these animals resulted in sustained plasma levels and CD4+ cell coating with humanized antibody for six weeks. These studies demonstrate the feasibility of chronic administration of CD4-specific mAb as a potential means of treating or preventing HIV-1 infection.